Various practices of using telephone facilities as a means of providing communications links for digital signal devices such as digital telephones, telemeter apparatus, digital computers and the like, are well known. Digital telephone facilities of the time division multiplex (TDM) type have been utilized to advantage to provide usable bandwidth of up to 64 Kb/s for example, for computer communications. Of course appropriate interface devices are required to transfer control and data signals between the operating formats of the telephone line of the telephone system and a communications port of the computer. One class of computers generally referred to as Personal Computers (PC) is exemplary, as such computers are often in an isolated situation where the only practical means of electronic communications is via a telephone connection. Some manufacturers of personal computers have made general provision for unforseen requirements by providing connection locations for additional circuit devices as and when required. In such an instant an appropriate telephone line interface circuit may reside in such a location and advantageously make use of support facilities already resident in the computer, such as power supplies for example. As the purpose of the telephone line is that of communicating at a distance from the telephone facility it is only natural the interface with the port of the computer be provided closely adjacent the computer and preferable by circuit means installed within the computer for this purpose. However in some instances the provided space may already be occupied or there may never have been any space provided in a particular module of PC. In such instances the interface device must reside independently of the computer and be connected thereto by a typical standardized computer communications port. Such ports are usually operable in one of several well known standards, RS-232, RS-422 and RS-423 for example. Furthermore such ports are not usually adapted to operating over distances similar to those distances over which typical telephone lines are usually operative. The telephone line may be operable in the standard analog format, the integrated services digital network format (ISDN), or in any of several proprietary formats. In any case the interface device is unlikely to have access to the computer as a source of operating power and therefor some other provision for operating power is required.
One example of an interface device is discussed in the U.S. Pat. No. 4,381,427 issued on Apr. 26, 1983 to W. E. Cheal et al. and titled "Subscriber Loop System for voice and data". In this patent, the interface device requires more operating current than is available to it via the telephone line. In order that a local utility power failure be of no consequence to the normal operation of the telephone system, a relay switchover feature is provided so that at least plain ordinary telephone service (POTS) may continue in spite of such power failure. In most modern telephone systems all seems well checks as to the operability of lines and telephone apparatus periodically occur. In Cheal et al's arrangement, a local power failure is prevented from disrupting the normal all seems well maintenance checks to the operating advantage of the telephone switch. Presently personal computers are usually adapted such that data calls may be originated or answered for example, via the computer key board, and hence the telephone apparatus as provided by Cheal is not essential.
Examples of commercially available integrated circuit devices used to provide this type of interface are Motorola MC1488, MC1489, 26LS31 and 26LS32. These devices obtain their operating power from the control switch of the telephone switching system. Recently, however, telephone switching systems, such as the Norstar system developed by Northern Telecom, have been proposed in which the current which is available to the remote device is too low to run such commercially available device. The Norstar system is described in TELESIS, Volume 16, Number 2, published by Bell-Northern Research Ltd. in 1989. The disclosure of that magazine is incorporated herein by reference. On the other hand, some commercial interface devices are available which operate with sufficient low current consumption but they can only operate at up to 20 Kbits/sec, which is much less than the 450 Kbits/sec required by some industry designs.
It is an object of the invention to provide an interface device which is remote from an associated telephone facility and which is independent of any local powering source such as a utility powered supply for its operation.
More specifically, it is an object of the invention to provide a telephone line powered interface device which couples information signals between a serial digital signal format of the telephone line and any of a plurality of standard port operating formats of a personal computer.